Complications due to infections during pregnancy are a significant risk factor for the emergence of schizophrenia (SZ) in the offspring. Animal models of prenatal immune challenge provide support for the idea that developmental immune abnormalities promote specific vulnerabilities of the disease. Conversion of the amino acid tryptophan to kynurenine (KYN) and its associated metabolites (collectively referred to as kynurenines) is one ofthe mechanisms activated during viral and bacterial infections. In particular, kynurenic acid (KYNA) is known to have neuroactive properties and is also elevated in the cerebral cortex of SZ patients. The purpose ofthe present project is to evaluate the involvement ofthe kynurenine system in the established animal model of prenatal infection. The central hypothesis is that activation ofthe enzyme indoleamine 2,3-dioxygenase (IDO) in response to the viral RNA mimetic poly l:C in the mother leads to increased production of kynurenines, including KYNA in the brain of embryos. This increase in KYNA will in turn be responsible for promoting microglia to adopt an alternative activated state also known as M2 activation. It is further hypothesized that this shift in activation state will be maintained throughout development, at least in a proportion of microglia, and that it will be exacerbated in response to stressors resulting in enhanced production of KYNA in the brain. In concert with the central hypothesis ofthe center grant, an increase in KYNA levels in the brain is believed to be responsible for cognitive impairments. We will test these hypotheses using rats prenatally challenged with poly l:C and subjected to a series of studies including a) documenting the trajectory of changes in the kynurenine pathway during pre and post- natal brain development and in relation to microglia activation; b) evaluating peripheral and central kynurenines and microglial responses to social stressors during peri-adolescence; and c) testing whether timed KAT II inhibition (reduction of KYNA production) prevents and/or reverses the neurobehavioral deficits seen in peri- adolescents. The present project will also involve the study of peripheral and placental cytokine and kynurenine responses in the mothers, as well as peripheral and central immune responses in the offspring.